This invention relates to an implantable medical device that delivers sufficient electrical energy to cardiac tissue to defibrillate or cardiovert tachyarrhythmias and thus restore normal sinus rhythm and, more particularly, to improved isolation of drive circuits for controlling discharge of high voltage capacitors providing a biphasic waveform shock.
In the field of automatic implantable arrhythmia control devices, the term xe2x80x9ccardioversionxe2x80x9d or xe2x80x9ccardioverterxe2x80x9d refers to the process of, and device for, discharging relatively high energy electrical pulses into, or across, cardiac tissue to arrest a life-threatening tachyarrhythmia. Cardioversion pulses may, or may not, be synchronized with a cardiac depolarization or rhythm and may be applied to arrest a malignant ventricular or atrial tachycardia or fibrillation with selectable or programmable pulse energy. The arrest of fibrillation by such pulses is referred to as xe2x80x9cdefibrillationxe2x80x9d (a form of cardioversion), and xe2x80x9cdefibrillatorsxe2x80x9d have been characterized as a form of cardioverter. In the context of the present invention, it is to be assumed that these terms are interchangeable, and that use of one term is inclusive of the other device or operation, unless specific distinctions are drawn between them. Current devices or implantable devices for the treatment of tachyarrhythmias, provide programmable staged therapies including anti-tachycardia pacing regimens and cardioversion energy and defibrillation energy shock regimens in order to terminate the arrhythmia with the most energy efficient and least traumatic therapies (if possible), as well as single chamber bradycardia pacing therapies. These devices provide a programmable energy, single polarity waveform, and shock from the discharge of a high voltage output capacitor bank through a pair of electrodes disposed in relation to the heart.
Commonly assigned U.S. Pat. No. 5,163,427 to Keimel discloses an implantable cardioverter/defibrillator system, which is capable of providing three defibrillation pulse methods, with a minimum of control and switching circuitry. The output stage is provided with two separate output capacitor banks, which are sequentially discharged during sequential pulse defibrillation and simultaneously discharged during single or simultaneous pulse defibrillation through a two or three electrode system.
Other cardioversion pulse wave shapes have been proposed in conjunction with a variety of electrode systems in order to achieve more efficient cardioversion, including bi-phasic or multi-phasic wave form shocks generated in rapid sequence and applied to the same or separate electrode systems as described in U.S. Pat. Nos. 4,800,833 to Winstrom, 4,830,006 to Haluska et. al., 4,998,531 to Bocchi, 4,953,551 to Mehra, 5,178,140 to Ibrahim, and 4,850,357 to Bach. Despite the additional complexity, it is expected that cardioversion may be achieved more rapidly after the onset of an arrhythmia and at lower current consumption. In order to achieve low current consumption, these stimulation therapy regimens require rapid and efficient charging of high voltage output capacitors from low voltage battery power sources as well as efficient sequential (or simultaneous) discharge of the capacitors through the electrode systems employed.
Generally, it is necessary to employ a DC-DC converter to convert electrical energy from a low voltage, low current power supply to a high voltage energy level stored in a high energy storage capacitor as substantially described in U.S. Pat. No. 5,265,588 and incorporated herein by reference in its entirety. A typical form of DC-DC converter is commonly referred to as a xe2x80x9cflybackxe2x80x9d converter which employs a transformer having a primary winding in series with the primary power supply and a secondary winding in series with the high energy discharge capacitors. An interrupting circuit or switch is placed in series with the primary coil and battery. Charging of the high-energy capacitors is accomplished by inducing a voltage in the primary winding of the transformer creating a magnetic field in the secondary winding. When the current in the primary winding is interrupted, the collapsing field develops a current in the secondary winding, which is applied to the high energy capacitors to charge them. The repeated interruption of the supply current charges the high-energy capacitors to a desired level over time. Such DC-DC converters are disclosed in the above referenced ""427 and ""588 patents wherein charging circuits are disclosed which employ flyback oscillator voltage converters which step up the power source voltage and apply charging current to output capacitors until the voltage on the capacitors reaches the programmed shock energy level.
In sequential pulse, multi-phasic systems, two or more output capacitors are charged and discharged through separate discharge circuits arranged in a bridge circuit configuration so that the sequentially generated shocks applied to the same electrode pathway(s) have opposite polarity. The discharge of the high voltage capacitors is typically effected by connecting the charged capacitors to the electrodes in discharge circuit paths through high voltage, high current conducting, Insulated Gate Transistors (IGTs) or metal oxide semiconductor field effect transistors (MOSFETs or power FETs), either employed alone or in electrical series with high voltage thyristers or xe2x80x9ctriacsxe2x80x9d. In the above referenced ""588, ""006, and ""427 patents, IGTs or power FETs are switched into conduction by dedicated drive circuits, which respond to low voltage control signals.
These low impedance, high current conducting switches are necessary to make and break the series electrical connection of the high voltage capacitors with the electrode/heart tissue load. The function of these switches must be tightly controlled to assure proper timing of the sequentially generated mono-phasic or biphasic shock impulses and to prevent destruction of the high voltage output circuit by the unintentional insertion of the switches directly across the high voltage capacitors. Noisy switch operation must also be suppressed. In order to electrically isolate the high voltage discharge circuits from the low voltage control circuits and microprocessor based control system, isolation transformers or optical isolators (opto-couplers) or capacitive coupling and common mode rejection circuits have been proposed. In the ""006, ""357 and ""531 patents, transformers are employed to couple discharge control signals to drive circuits. As stated in the ""140 patent, such transformers are bulky, and the transformer cores are susceptible to saturation by external magnetic fields.
The optical isolators and driver circuits employed in the ""427 patent do not suffer from these drawbacks but still take sizable hybrid circuit volume, are costly, consume battery power, and have potential catastrophic failure modes.
Accordingly, one aspect of the present invention to provide a highly energy efficient, cost effective and compact circuit for driving high voltage switches in the output circuit of an implantable automatic cardioverter/defibrillator.
Yet another aspect is to provide a driving circuit for the high voltage switches of a cardioversion/defibrillation pulse generator that improves isolation between high and low voltage components and prevents transients from affecting the operation of the switches.
It is a further aspect to provide a cardioversion/defibrillation pulse generator that isolates the battery power supply for the low voltage control system from a separate low voltage power supply for the low voltage drive circuits of the high voltage switches in the high voltage output circuit without the addition of bulky components.
The above aspects and attendant advances are achieved in the context of a battery powered cardioverter or defibrillator employing a DC-DC converter for charging high voltage output capacitors and for delivering biphasic cardioversion or defibrillation pulses through a bridge circuit including high and low side drive circuits under the control of a microprocessor controlled arrhythmia detection system. Upon the detection of an arrhythmia and the selection of cardioversion/defibrillation therapy, the charging of the high voltage output capacitors is commenced and the capacitor voltage enables a regulated voltage source for the high and low side drive circuits for the high power IGTs of each branch of the high voltage bridge output circuit.
Upon reaching full charge, the microprocessor provides first and second, biphasic pulse width defining, control signals in succession to separate inputs of each low side drive circuit which either provide a trigger signal to a high side drive circuit or a gate control signal to a low side IGT so that only one branch of the bridge circuit is enabled for conduction and discharge of the high voltage capacitors through the patient""s heart during each phase. The respective high side drive circuit is triggered into producing a high side IGT gate control signal, and both IGTs of the branch are switched rapidly into conduction for the pulse width defined by the duration of the respective control signal.
In accordance with a particular aspect of the invention, high voltage switching transients are suppressed from re-triggering or otherwise affecting operation of the drive circuits. In this regard, discharge circuit means for delivering voltage stored on capacitor means to the heart in a discharge mode of operation and in response to a discharge control signal further comprises high voltage discharge control switch means operable in response to a switching signal for connecting and disconnecting said high voltage capacitor means with the heart for discharging the capacitor means through the heart during the period of connection, voltage regulating means coupled to the capacitor means for sensing the voltage level stored on the capacitor means and for generating a regulated voltage upon charging of the capacitor means to a predetermined voltage level, drive circuit means powered by the regulated voltage and triggerable at an input terminal in response to said discharge control signal for providing the switching signal to the high voltage discharge control switch means, and means for inhibiting transient high voltage signals generated during switching of said high voltage switch means and coupled back to the input terminal of the drive circuit means from re-triggering the drive circuit means.
The inhibiting means preferably comprises monolithic isolation circuitry that uses an isolated output current replicator of an input current in an isolated input current loop. The current replicator comprises an input current loop and an output current loop that are isolated from one another to inhibit high voltage transients in the output current loop during delivery of the cardioversion/defibrillation shock from being reflected or conducted into the input current loop and to the low voltage circuitry potentially causing damage.
The novel elements believed to be characteristic of the present invention are set forth in the appended claims. The invention itself, together with additional objects and attendant advantages, will best be understood by reference to the following detailed description, which, when taken in conjunction with the accompanying drawings, describes a presently preferred embodiment.